The present invention relates to a system for de-energizing an electrical device, connected to an alternating current electrical power supply, when undesirable conditions, in addition to undesirable leakage currents in the power cord, develop within the electrical device. Such undesirable conditions may include among others, flames, too high of a temperature, a pressure deviating too far from normal, the presence of smoke or particulate matter or, the presence or absence of light or of a light opaque member within the device. Still another undesirable condition is the presence of an electrical potential on metal parts, such as the frame of the electrical device which could cause a leakage current.
The protection system of the present invention provides additional protection, not provided by prior systems, for electrical appliances and tools. In addition to providing protection from electrical leakage currents, and protecting a user from the hazard of electrical shock, it includes additional sensors for detecting and providing protection with respect to flames, smoke, excessively high temperatures, and abnormal pressures in the appliance or tool.
More particularly it relates to an enhancement of appliance leakage current interrupters (ALCls) and equipment leakage current interrupters (ELCls), to provide additional protection to a leakage current protected electrical device. More generally, it relates to an enhancement of any Leakage Current Protection Device as set forth in Underwriters Laboratories, Inc. Reference Standard UL943A, which includes not only ALCIs and ELCIs, but also immersion detection circuit interrupters (IDCIs). Whenever the term GFCI or ground fault circuit interrupter is used herein, it is intended in the broader sense of any leakage current protection device, where applicable.
In accordance with this invention, the protection provided against leakage currents and arcing (generally referred to as arcing currents herein), either separate from or in connection with a GFCI, to an electrical device power delivery cord, is extended to the electrical device which is connected to a power supply through the power delivery cord. The extended protection encompasses not only leakage currents and arcing, but also other undesirable conditions which may occur in the electrical device. Typical of the electrical devices which may be protected in accordance with this invention are electric heaters, air purifiers, room air conditioners, water distillers, and such kitchen appliances as toasters, toaster ovens, bread makers, and coffee makers. The extended protection upon detection of unsafe or undesirable conditions indicated by abnormal light or source of light, temperature or pressure conditions, or the presence of flame or smoke will result in disconnecting the electrical device form its power source.
One embodiment of the protection system of this invention is for use with two wire electrical devices such as an appliance or tool, which receives electrical power through a power cord, which includes an unshielded power line, and an unshielded neutral line. As described in co-pending application Ser. No. 08/682,957, entitled: xe2x80x9cElectrical System With Arc Protectionxe2x80x9d, which application is assigned to the assignee of this application, and which application is hereby incorporated by reference, a sensing lead, substantially electrically isolated from the power and neutral lines and the electrical load of the appliance, is provided in the power cord. A sensing lead integrity checking device, such as a neon glow tube or a switch, may be connected between the sensing lead and the power line. The integrity checking device connects the power line to the neutral line through the sensing lead. The impedance of the integrity checking device is increased by substantial additional resistance, so that the current flow is very small, well below the pre-determined magnitude that would initiate response of the arcing current protection arrangement. Thus, although the integrity checking device does provide a connection between the sensing lead and the power line, the power line and the sensing lead are only connected for a brief integrity checking period through a switch, or the total impedance between the power line and the sensing lead is so high that, for practical purposes of the arcing current protection arrangement, the sensing lead is isolated from the power line and the appliance or tool load.
A rectifier, such as a diode rectifier circuit, may be connected between the power line and the neutral line to provide DC power to a circuit opening device, such as a solenoid which actuates normally closed contacts in the power line. In most situations, it is also desirable to open the neutral line. In such situations, a second pair of normally closed contacts in the neutral line are actuated by the solenoid.
Energization of the solenoid coil may be controlled by a suitable switching device, such as a silicon controlled rectifier (SCR) having the anode-cathode circuit connected in series with the solenoid coil. The gate of the SCR coil may be connected to the cathode through a suitable resistance that will bias the SCR to a conducting state when current flow of a pre-determined magnitude passes through the resistance, thus providing a current responsive arrangement. The sensing lead is also be connected to the gate of the silicon controlled rectifier, usually through a limiting resistor. Of course, with a different type of circuit opening device, the sensing lead would be connected to the control circuitry therefor.
When an arding current above a predetermined magnitude is produced on the sensing lead, such as by detecting an arc resulting from a breakdown of the insulation separating the power line and the neutral line, the SCR will be gated to a conducting state and the solenoid will be energized to open the power line and, usually, the neutral line as well. Typically the leakage current detection level is set between a range of 250 micro-amperes and 5 milliamperes.
In accordance with this invention the sensing lead is electrically connected to the metal housing or frame of the electrical device. If there is a dielectric or insulation failure between the metal housing or frame and the power or neutral line, and a leakage current, tracking or arcing occurs with respect to the metal housing or frame of the electrical device, it will flow through the sensing lead to cause the SCR or other suitable switching device to be gated to a conducting state. Thus, the solenoid will be actuated by energization or de-energization, depending on the circuit configuration, to open the power line and usually, the neutral line as well. This embodiment of the arcing current protection arrangement may be used independently of a GFCI, although a GFCI would normally be included somewhere within the electrical system to protect against ground fault currents.
In a variation of the first embodiment of this invention a supplemental coil is placed on the core of the differential transformer of a GFCI. The sensing lead is connected to this supplemental winding, which is in turn connected to the neutral line through a suitable resistance. If an arcing current above a pre-determined magnitude should occur in the sensing lead, the current flow through the supplemental winding creates an imbalance in the differential transformer, which in turn produces a trip signal in the secondary winding of the differential transformer. The trip signal in the secondary winding initiates actuation of a circuit opening device to break the power line and protect against a hazardous condition that might be created by the arcing current.
The number of turns in the supplemental winding may be varied to adjust sensitivity to arcing currents. Thus, the magnitude of the arcing current at which the circuit opening device will be actuated may be adjusted in accordance with the desired protection. Typically the current detection level is set in the range of 250 micro amperes to 5 milliamperes. The resistance in series with the supplemental winding may also be adjusted to establish the predetermined magnitude of arcing current at which tripping is desired. Again, the metal housing or frame of the electrical device is connected to the sensing lead. As set forth with respect to the previous embodiment, if a current above a predetermined magnitude occurs in the sensing lead, due to a current flow between the metallic housing and the sensing lead, the trip signal in the secondary winding initiates actuation of a circuit opening device to break the power line and protect against the hazardous conditions that might be created by the arcing current.
Further protection is provided to an electrical device in accordance with this invention by de-energizing the device when an undesirable condition occurs in the device. The further protection is provided by causing a circuit to be completed to the sensing lead from the power line or from the neutral line in response to the occurrence of an undesirable condition in the electrical device. Various parameters, indicative of an undesirable or unsafe condition in the electrical device, may be detected, and if detected cause a circuit to be completed between the sensing lead and the power line or neutral line. Amongst the variety of parameters which may be sensed are light, temperature, flame, pressure and smoke. In accordance with this invention, simple low cost sensors may be used as detectors. Various arrangements may be used to complete the circuit between the sensing lead and the power line or neutral line. In one embodiment of this invention a normally closed switch connected in a circuit connecting the sensing lead to the power line or neutral line is held in an open position by a mechanical restraint. Upon the occurrence of an undesirable condition, the mechanical restraint is released. Closing of the normally closed switch contacts causes a current to flow in the sensing lead, which results in disconnecting the electrical device from the power source. Amongst the other low cost sensors which may be used in accordance with this invention are cadmium sulfide light detection cells, a combination of a light emitting diode and light sensitive semiconductor switch, and a pressure sensitive switch.
As an example of this embodiment of the invention, a switch normally biased to a closed position is held in an open position by a force applied through a tape formed of a heat sensitive material. When an undesirably high temperature is reached in the electrical device, the tape will stretch or disintegrate, thereby allowing the mechanical bias on the normally closed switch to move the switch contacts to the closed position. A resulting current flow through the normally closed switch contacts and the sensing lead will cause the electrical device to be disconnected from the power source.
A second embodiment of the protection system of this invention is for use with electrical devices such as appliances or tools having three lines, that is power, neutral and ground. It may also be used with three wire or line devices having a metal sheath or cover on the power cord. A ground fault current interrupter circuit with arcing protection for such three wire or line devices is set forth in U.S. Pat. No. 4,931,894xe2x80x94Legatti, which patent is assigned to the assignee of this application. U.S. Pat. No. 4,931,894 is hereby incorporated by reference. An appliance or tool having three lines would normally have its metal housing or frame connected to the ground line such that arcing to the metal housing or frame would be detected by the ground fault current interrupter circuit. Thus, a sensing lead need not be connected to the metal housing or frame to detect arcing thereto, as in the first embodiment. However, in accordance with the second embodiment of this invention, a sensing lead is provided for the purpose of providing a signal, representing an undesirable condition in the electrical device, to the leakage current protective device connected to the electrical device to open the power circuit to the electrical device. As in the first embodiment, the sensing lead may be connected to actuate a switching device such as a silicon controlled rectifier, or it may be connected to a supplemental coil on the core of a differential transformer of a GFCI.